Nobody wears oxygen or forced ventillation when flying ag. A few, very, very few, might occasionally use a respirator when flying, but I've never seen it. Especially in the summer, the heat can really build up under a greenhouse canopy (I've measured 180 degrees in august)...throwing on a respirator is not only foolish, but dangerous.
I wear a HGU-55/P, and had mine done in Kevlar. It's a personal option. The 55/P is probably the lightest headset out there. A lot of guys I know use the heavier SPH-4 or SPH-5 helicopter helmet. These tend to circulate more air and are more open, though they stick out far enough on the sides that I find it's too easy to keep banging the canopy just by turning your head.
The idea behind the helmet is obviously protection in the event of studden stoppage somewhere, but more than that, it's inflight protection because my head gets bounced off the canopy not too infrequently...get some moderateor severe turbulence, and everything gets very mobile in the cockpit, including the pilot.
FlightSuits (I forget what their new name is) makes good products, but you can also go with other firms such as Government Sales and get better prices. You can go surplus, but I do not recommend that with a helmet. Nomex gear, fine, but I strongly recommend a new helmet.
A new helmet can be made to order and should be. For this type of work, a new helmet should be a milspec helmet and definitely not a motorcycle type helmet. I recommend avoiding traditional fiberglass helmets; get kevlar. New helmets can be had in standard sizes, but I recommend a custom fit helmet, especially if you'll be wearing it very much. I also recommend going with a temper foam that adjusts to your head, rather than velcro or foil spacers inside
You'll have a choice between the "elephant ears" style earcups, or the traditional hard david clark style earcups, I recommend the david clark style, but go with oregon aero ear seals. Weather you're flying radial or turbine, they still provide better noise protection. You can get ANR in the avionic portion of the helmet either way, but it will work better if you don't use the elephant ears. The elephant ear installation is more comfortable, some say, but with the use of the oregon aero ear seals, I don't know that I'd agree.
Buying a new custom helmet also allows you to fit your choice of avionics, order your choice of visor, etc. All that can be had after market, too. However, once you've had one or two hot days in the cockpit, you'll come to appreciate why a one-owner helmet is the way to go, especially after you've been sweating into it like a stuck pig.
Get the longer plug in cord, or you'll only need an extention adapter...ask for at least six feet of cord, and you'll be doing well.
One problem with the HGU-55/P helmet, if it can be called a problem, is the exterior visor. The older HGU-26 and 33 helmets used an external visor cover...the 33 is the type of helmet you see in movies like "top gun." This uses a visor inside a plastic housing, sometimes two different visors for night and day. The assembly is bulkier and heavier. The 55 uses a visor that's attached with elastic straps, and works well, but isn't covered or protected by a housing. It can get scratched. The soloution is to keep a leather or vinyl cover velcroed over it when not in use...which works.
I keep my cover in my left flightsuit leg pocket; it comes off when I put on the helmet in the cockpit, and goes back on when I take the helmet off at the end of the flight. However, it's still possible to scratch up the visor (which costs about fifty bucks) when stick my head out of the cockpit during taxi, or even when looking around while maneuvering, if the visor is pushed up on top of the helmet.
I can't wear glasses comfortably with the helmet on (about the only glasses you can wear with them are military straight bayonet glasses, and in a hot cockpit, it's hard to mop your eyes or face with those pinned on), so I usually wear the visor down, and I use a dark grey visor in lieu of sun glasses. It helps increase contrast, which can really be important with fine obstacles such as powerlines, standpipes, trees, snags, etc. It's also really critical when working in a smokey environment, close to rocks, etc, where depth perception is important.
If you need economy, you can get helmets from various sales agents such as Government Sales or Flight Suits Unlimited as a special. Sometimes racing teams or special units over order on the helmets, and they have extra at a discount price. The same at flight suits can be said for the nomex. A new flight suit runs about two hundred twenty dollars, and you can get them for a hundred or less, if you want new, on special. These are usually specially made items made to order, often in standard sizes in special colors. If you want an indian orange flight suit, like military Test used to wear...FSU is about the only place to go any more.
One company switched to Indian Orange a few years ago after losing a pilot. He died, but died late in the game when he should have been found. He was wearing sage green nomex, and searchers didn't locate him until it was too late. The company immediately switched to indian orange. Everybody looked like they were going hunting, but it's a lot more visible, and a lot safer, even on a busy ramp with lots of props turning.
For what purpose are you seeking a helmet? That will also influence what you buy. If I were seeking another helmet right now, I would probably go for a HGU-84.